#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>

int main()
{
    int  n;
    double sum = 0;
    int yi = 0;
    int i, s = 1;
    scanf("%d", &n);
    for (i = 1; i <= n; i++)
    {
        yi += s * (2 * i - 1);
        sum += 1.0 / yi;
        s = -s;
    }
    printf("%.3lf\n", sum);
    return 0;
}